Garden School:

Garden School:
Teaching this week: Rose pruning (as always!) and leaf mold.

Monday, 19 March 2018

Vine Weevils and what to do about them

A question came in yesterday, concerning the dreaded vine weevils. Before I get to the actual question, here's a bit of background.

What are they, and why are they dreaded?

Well, they are a small, crawling insect which eats the leaves of plants, then lays eggs on the soil around the plant. These eggs hatch into nasty, greedy, little grubs which eat the roots. This causes the plant to die. And this is why we hate them so much.

They have a real taste for plants in pots: you do find them "loose" in the garden, but not that often - however, most people with plants in pots will, at some point, find they have an infestation of vine weevil.

So, Know Thine Enemy.  Here's a picture of the little blighter:

Note the ribbed appearance of the back end, which is usually black but can have splotches of buff/brown/orange: note the jointed carapace, the snouty snout, the long probing bits, the sharply-angled legs.

This is a female - they are all female - and they are born ready fertilised, so they start laying eggs as soon as they are adult.

The eggs are tiny things, they hatch out into:

... these nasty C-shaped grubs. Creamy white colour, note the brown head-cap.

All the books will say C-shaped, by the way, but it useful to know that they sometimes appear to be quite straight. Poking them will usually make them curl into the C shape, though.  These are the ones that do the damage to the roots.

The C-shaped grubs eventually pupate, so sometimes you find these weird things (left): they have lost the brown cap, and have developed rudimentary legs and snouts.

I always think they look a bit like chess pieces at this stage. If you find them, they are still moving, but slowly.

This is the damage which the adults do: "notched leaves".  They sit on the edge of the leaf and chomp out a big piece.

Can be confused with the damage caused by leaf-cutter bees: but the bees do a very neat circular hole, very regular: vine weevils make ragged notches.

Why are they such a problem? Because those grubs will devastate plants in pots: they will eat and eat and eat until the roots are all gone. If you don't spot them, they pupate, hatch, and lay more eggs. Thus the cycle continues, and once you have them, they are very hard to eradicate.

How do you get them?

Usually, they are brought into a garden on an infected plant. Often, regretfully, bought from an otherwise respectable garden centre or nursery... who are the very people best equipped to spot them, and - as plants are their "product" - who should be obliged to ensure that all plants are weevil-free.  However, they are not, and this is why, in a perfect world, we would keep new plants in a quarantine area until we are sure they are not infected with not just weevils, but any other creepy crawley and/or disease.

Of course, in a perfect world, the garden centres and nurseries would do their job properly and ensure the plants are not infected....

Vine weevils can't fly (thank the lord!) but they are very good climbers, and tireless walkers, so in a garden centre or nursery, where the plants are all crammed together in pots, they quickly spread and infect the entire place.  Then, when you innocently bring an infected pot home, they quickly spread to all your other pots, curse them.

How to spot them:

The obvious sign is the notches leaves where the adults have been feeding, but that's not always visible: you could have a pot which  is heaving with grubs, or has just had eggs laid in it, but on which there are no longer any adults, so you won't see any damaged foliage.

Usually, the first people know about it is when the plants start to die. Or if you go to move a plant, and rather lazily pick it up by the plant instead of by the pot (be honest, we've all done it!) and the plant comes off in your hand, leaving the pot and the soil (and the shattered remnants of the roots) behind.

When you tip out the pot, you will either find little air pockets, usually containing a grub, right up against the side of the pot: or, if you shake the plant to loosen the soil, you'll find the white grubs falling out from the centre of the rootball.

What to do:

As soon as you find the first sign - the first notch, the first rootless plant, the first time you tip out a pot and find grubs - you must take action.

Check every pot that you have. Every single one! If you have small pots, or plastic/terracotta ones up to about 3litres, here is the regime:

1) Move them all to one end of the patio, so you can go through them methodically, one at a time.

2) Shake them upside down over a tray, or over a hard path (concrete or tarmac) to see if any adults fall out of the plant. Any that you find, crush mercilessly with your boot, or cut in half with scissors/secateurs.

3) De-pot: turn the pot upside down, knock the plant out of the pot, and check the rootball. If you see white grubs, put it to one side, on the "infected" pile. Feel free to kill any and every white grub that you find: you can squish them by hand - it's ok to wear gloves - you can crush them with the back of the trowel (be warned, they squirt), you can chop them in half with scissors/secateurs/knife, you can drop them into a jar of salty water and leave them to drown (takes days, and when would you be sure?) you can even put them out for the birds, but only if you put them in a dish that they can't climb out of.

4) If you don't see grubs on the outside, pull the plant fairly firmly, to see if it is still properly rooted. If the plant appears to be firmly rooted, put it back in the pot and put in the "probably ok but we'll check again later" pile.

5) If it "gives" at all, then take the pot to a potting bench or tray, and shake off the soil until you can see the roots clearly. Any white grubs, put it in the "infected" pile. If none, wash the remaining soil off the roots by dunking the plant into a bucket of water and swishing it about. This plant is now clear - no adults, no grubs, no eggs - so it can be potted up again, using clean fresh compost. Discard the compost/soil that  you removed: don't put in on the compost heap, as this just spreads them all round your garden: instead,  put in your council wheelie-bin for garden waste if you have one, and if not, collect it all and take it to the tip.

6) Having worked through all your pots, look at how many were found to be infected: if it's only a few, then the "probably ok" ones can be returned to the patio display. If there are a lot of infected ones, then go back to the "probably" pile and go through stage 5 for each one: de-pot, shake off the soil, wash off the soil, repot in clean compost, dispose of the soil.

As for the "infected" ones, go through stage 5 with them but if, when you shake off the soil, there are hardly any roots left, the throw the plant away: unless it is really precious to you, there is no point wasting a year trying to revive it, it is better to go out and buy new ones.

And if you do, don't forget to check them all before you bring them back into your freshly de-contaminated garden: do stages 2, 3 and 4, preferably before you get to the tills.

If you ever find vine weevils or grubs in plants at a nursery or a garden centre, take it straight to customer service and demand to see the manager. Tell all your friends not to shop there, until you are certain that they have dealt with the problem.

"But I have really big pots."

If you have large pots, such that you can't tip them upside down, then spread an old sheet around the base of the plant, and vigorously shake the foliage, ruffling it with your hands, to see if any adults fall out.

Then, see if you can dig around the base of the plant: if the soil is compacted or full of roots, then you won't be able to do a physical check, so your only option is to go for chemical or biological warfare.

Chemical warfare: There are products you can buy to kill the grubs: you dilute it, water it on, and after several weeks it will have killed any grubs. Follow the instructions very carefully, especially in relation to time of year/temperature.

Biological warfare: You buy a pack of live organisms which you water onto the soil: these organisms find their way into the grub, where they replicate and kill the grubs. They can be incredibly effective, BUT they can sometimes be completely ineffective.

Why the difference? If you read the instructions and follow them, only applying the nematodes at the right time of year, within the right temperature range, and before they die, then they will kill all the grubs. It all goes wrong if you keep the pack for too long before using it: or if you apply them at the wrong time of year, or when it's too cold.

Additionally, they don't kill the adults or the eggs, so you will have to apply them again - in spring, and again in autumn, as per the instructions on the pack -  in order to break the cycle.

So, let's get onto Sue's problem: she wrote to me, saying that she found notched leaves for the first time in her garden last year, then found adults, so she applied nematodes in autumn. This spring ("Spring? Have you seen the snow outside??!") she de-potted a rose, planning to plant it out in the garden, only to discover grubs in it.  The question was, is there any way of getting rid of these things for once and for all.

The answer for Sue and anyone else with the same problem is yes - but it will take a bit of work.  Having gone through all your pots and shaken out the adults, shaken out/washed off the grubs and eggs, repotted everything and applied chemicals and/or nematodes as you saw fit, you then have to do the following:

1) Constant vigilance. Once you have an infestation of vine weevil, you have to be continually on the lookout for them. Inspect leaves every week for damage, gently tug on potted plants to ensure they are still firmly rooted, and once or twice a  year, go through all your pots and de-pot the plants to check their rootballs.   This is a good thing to do anyway, as it gives you a chance to see if they are becoming rootbound, or are too dry/too wet etc.  Go out after dusk with a torch, as that is when the adults are likely to be walking around.

2) Be harsh: if you find infected plants, if you cannot be bothered to completely remove ALL of the soil by shaking, and by washing it off, then you must discard infected plants and buy new ones.

3) Always check new plants very carefully, and if possible, quarantine them for a few weeks so you can check up on them before introducing them to your garden.

4) Consider raising your pots onto benches or stands, and applying grease bands to the legs: this is sticky stuff, rather like flypaper but on a roll, which you wrap around the trunks of trees to catch all the non-flying bugs. It works just as well on bench legs, although it's a horrible fiddly, sticky job to get them in place. But, if you are desperate, it's worth doing for a year, to break the cycle of adults-eggs-grubs-pupae-adults.

So there you have it, vine weevils in a nutshell (horrible mental image, why did I say that?): if you get them, it's probably not your fault, but to get rid of them takes constant vigilance and a degree of persistence - but it can be done!